2j6 the formation 



267. Agents of migration. In the last analysis, however, the possibility ot 

 migration depends upon the action of distributive agents ; in the absence of 

 these, even the most perfect contrivance is valueless, while their presence 

 brings about the distribution of the most mimobile form. In short, migra- 

 tion depends much more upon such agents than upon mobility, however per- 

 fect the latter may be. It is, moreover, evident that the amount and extent 

 of migration will be determined primarily by the permanence and forceful- 

 ness of the agent, as indicated by its ability to bring about transportation. 

 Finally, as will be shown later, the direction and rapidity of migration de- 

 pend directly upon the direction and intensity of the agent. 



Migration results when spores, seeds, fruits, offshoots, or plants are moved 

 out of their home by water, wind, animals, man, gravity, glaciers, growth, or 

 mechanical propulsion. Corresponding to these agents, there may be recog- 

 nized the following groups : 



1. Water, hydrochores. These comprise all plants distributed exclusively 

 by water, whether the latter acts as ocean currents, tides, streams, or surface 

 run-off. In the case of streams and run-off, especially, mobility plays little 

 part, provided the disseminules are impervious or little subject to injury by 

 water. Motile plants, or those with motile cells, which belong entirely to 

 this group, may be distinguished as autochores, which correspond closely to 

 mastigospores. 



2. Wind, anemochores. This group includes the majority of all permo- 

 bile terrestrial plants, i. e., those in which modifications for increasing sur- 

 face have been carried to the extreme, or those which are already permobile 

 by reason of the minuteness of the spore or seed. Saccate, winged, comate, 

 parachute, pappose, plumed, and, to a certain extent, awned seeds and fruits 

 represent the various types of modifications for wind-distribution. 



3. Animals, zoochores. Among terrestrial plants, dissemination by at- 

 tachment represents essentially the same degree of specialization as is found 

 in wind-distributed plants. The three types of contrivances for this purpose 

 are found in spinose, hooked, and glandular fruits. Dissemination by 

 deglutition and by carriage, either intentional or unintentional, though of 

 less value, play a striking part on account of the great distance to which 

 the seeds may be carried. Dissemination by deglutition is characteristic 

 of sarcospores, and distribution by carriage of creatospores. 



4. Man, brotochorcs. Dissemination by man has practically no connec- 

 tion with mobility. It operates through great distances and over immense 

 areas as well as near at hand. It may be intentional, as in the case of 

 cultivated species, or unintentional, as in thousands of native or exotic 

 species. No other disseminating agent is comparable with man in respect 

 to universal and obvious migration. 



